In many radar applications it is necessary to form two-dimensional images of targets such as ground vehicles, aircraft, ships, and so forth. Resolution in one dimension is provided by the range resolution inherent in the transmit waveform, and resolution in the other cross range dimension is provided by Doppler resolution. The principles are widely applicable and widely applied. Synthetic aperture radar (SAR) forms two-dimensional images in range and cross range with the cross range dimension utilizing the motion of the platform for Doppler resolution. Inverse synthetic aperture radar (ISAR) is accomplishing the same objective with the cross range dimension utilizing the motion of the target being imaged. In other applications the same principle of Doppler resolution is used, even though no specific name has been given to the process. The very same principles also if, instead of forming images, the processor uses range and Doppler resolution to resolve specific scatterers on the target, and then measures the separation of these scatterers in order to obtain target dimensions. All these multi-dimensional imaging techniques require high range resolution (HRR).
Range resolution (Δrs) is directly related to bandwidth:Δrs=c/(2*B)where c is the speed of light and B is the bandwith or frequency excursion of the transmit signal. Modem sub foot imaging systems must therefore transmit and receive an equivalent bandwidth of greater than 600 Mhz. Six inch resolution requires greater than 1.2 Ghz of equivalent bandwidth. The transmitter must be capable of generating this wideband signal and transmitting it without distortion. On receive, the wideband signal must be downconverted and sampled at or above Nyquist for processing. For the six inch case, Nyquist at baseband is 1.2 Ghz. Direct methods exist that operate with the instantaneous bandwidth being the resolution bandwidth. Two direct waveforms are currently used in obtaining high resolution 1) Direct Short Pulse and 2) Chirped Pulse Compression.
In direct short pulse systems the time bandwidth product of a rectangular pulse is ˜1.Bτ˜1where τ is the pulsewidth. The range resolution of a direct short pulse is therefore:Δrs=(c*τ)/2High range resolution using short pulses is possible with both noncoherent and coherent radars. Magnetron transmitters in noncoherent radars can be turned on and off rapidly enough to generate pulses with a minimum pulsewidth of ˜50 nanoseconds. This has a corresponding range resolution of 24.6 ft. Peak and average power capabilities of a magnetron based system is capable of providing performance at longer ranges however without the resolution. To achieve a range resolution of 1 foot and six inches requires a pulsewidth of ˜2 and 1 nanoseconds respectively. Impulse generators have been used to generate these extremely small pulsewidths. These small pulsewidth can be applied to a High Power Amplifier such as a Traveling Wave Tube Amplifier (TWTA), however even with the higher peak power, the average powers achievable limits the long range performance. For this reason the sub foot direct pulse configurations have been primarily used in Radar Cross Section diagnosis's configurations.
Chirped pulse compression allows a radar system to transmit a pulse of relatively long duration pulse (microseconds) at a higher peak power pulse to simultaneously attain the range resolution of a short pulse and the high average power of a long pulse transmitter. Fine range resolution is achieved in pulse compression systems by coding the RF carrier of the long pulse to increase the bandwidth of the transmit signal. When the reflected signal is received, the coded waveform is applied to a matched filter that compresses the transmitted pulse. Most airborne radars use linear Frequency Modulation (LFM) where the frequency of the transmit signal varies linearly with time. For example, the transmit frequency of one radar varies by 185 MHz over a 40 microsecond pulse. The receiver collects reflected pulses, which are 3.25 nm long and compresses them into an effective pulse width that is 5.4 nanoseconds. This corresponds to a resolution of 0.8 m. The 10 kilowatt peak power transmitter of the transmitter provides an average power of 200 watts on targets out to 160 nm. If a magnetron-based radar system could transmit a 5.4 nanosecond pulse, it would have to output 74 MW of power to achieve the same average power on target. It would not be practical to fly such a transmitter on a aircraft because of the larger size, prime power consumption and cooling apparatus. As a result of pulse compression, the 25 foot resolutions achieved with magnetron systems have been reduced to less than 1 m. Chirped pulse compression systems can achieve sub foot resolutions by transmitting and receiving the necessary bandwidth in the coded waveform. However, as mentioned previously, the transmit and receive hardware must support the instantaneous bandwidth. At X-Band the ratio of bandwidth to carrier frequency to achieve sub foot resolution is >10%. This pushes the hardware up against technology constraints and results in an expensive and complex radar system.
Stretch processing of linear FM pulses originally developed by W. Caputi, reduced the complexity on the receiver side of the radar. The stretch concept provides HRR by transmitting a linear FM pulse of the necessary bandwidth just like a chirped pulse compression system. On receive the returns are down-converted in frequency with a frequency modulated Local Oscillator signal of identical FM slope as the transmit signal. The corresponding down-converted signals are fixed frequency pulses, the center frequency of the pulse depends upon the relative range of the range gated scene. The IF signals are sampled and converted to the frequency domain providing a range profile of the scene. This technique does reduce the complexity of the receive path however the transmit path remains complex and the technique is not extendable to larger range swaths.
To bypass the complexity, cost and technological limitations of achieving subfoot high range resolution with transmit signals containing the necessary instantaneous bandwidth, engineers have developed a technique to synthesize the HRR using multiple narrower band signals of different frequencies. This technique, known as the stepped frequency waveform (SFW) consists of a sequence of pulses transmitted with fixed uniform pulse to pulse frequency change (ΔF). The number of pulses (N) required is a function of the desired resolution and the pulse to pulse frequency change (ΔF).Δrs=c/(2*N*ΔF)The SFW process is not a single look process like the direct methods described above. It requires the transmission and reception of multiple pulses. The total time to transmit the necessary pulses to synthesize the HRR waveform is simply the radar PRI times the number of pulses. Along with the simplicity of the technique there are many limitations that limit its usefulness. Conventional Step Frequency processing does not handle moving targets either actual motion or motion due to scene rotation about a fixed point in a Spot SAR or ISAR image. It requires a large number of individual frequencies to permit acceptable sample weighting to achieve low range sidelobes after the DFT. The coarse range samples are unmodulated pulses. The large number of frequencies required may mean that there are insufficient samples to permit target Doppler Frequency measurement and there may be a multitude of adjacent channel self-jamming situations. DFT/FFT processing requires a constant time step between transmissions. This will exacerbate any target motion